The present invention is related to rare-earth chromites that are used as electrical components.
Conventional rare-earth chromites are useful in a number of electrical applications because their electronic structure makes such chromites electrically conductive. In addition, the refractory nature of the materials at high temperatures makes them useful for service at high temperatures under relatively severe conditions.
Typical applications for conventional rare-earth chromites are resistance heating electrodes or electrodes for the magnetohydrodynamic (MHD) generation of electricity. In such applications, it may be preferable to alter the conductivity of the chromite and several prior art methods have been developed.
The method of primary interest herein is that of substituting other metallic ions for those normally within the chromite structure. Generally, such substitution is termed "doping." The effect of the doping depends on the metal ion substituted by introduction into the crystalline structure, the ion that is eliminated from the crystalline structure by the substitution and the amount of metallic ions being substituted.
Rare-earth chromites have the Perovskite structure, normally designated by the formula ABO.sub.3. A and B in this formula for Perovskite relate to its crystalline structure. Perovksite has an orthorhombic crystalline structure with the metal ions arranged in the structure either at the corner positions in the crystal lattice (the A-sites) or the body-centered positions (the B-sites). The oxygen ions are at the face-centered sites of the orthorhombic crystalline structure. In the case of the present invention, the Perovskite is a chromite, so B in the formula is chromium. The rare-earth chromite of this type is also conventionally designated as RCrO.sub.3, where R is the rare-earth element.
The electrical conductivity of rare-earth chromites results from the interaction of the unfilled d orbitals of the chromium ions. If d orbitals overlap, a conduction band is formed. A wide conduction band produces an electrically conductive material. The modification of the electronic structure of the chromite is the primary object of doping procedures.
The doping of rare-earth chromites with bivalent metal ions appears to cause a charge imbalance in the Perovskite lattice, thus forcing a valence change in the chromite from three to four to maintain electrical neutrality. As a result, an electron is transferred into the conductivity band. Using this procedure, the controlled valency technique leads to the formation of a P-types emiconductor.
It is known in the art to modify the electronic properties of the rare-earth chromites by doping them with metallic ions that substitute at the A- and B-sites of the Perovskite lattice. The introduction of calcium ions to rare-earth chromites is disclosed in U.S. Pat. No. 3,630,968 to Hamano et al., and the introduction of magnesium or strontium ions is disclosed in U.S. Pat. No. 3,974,108 to Staut et al. The substitution of these ions, as shown in the prior art, has the desired effect on the electrical properties of the rare-earth chromites.
Typically, rare-earth chromites such as lanthanum chromite are prepared by dissolving lanthanum oxide in an aqueous chromium trioxide solution, drying the solution, and calcining the resultant product in an oxidizing atmosphere.
In the prior art, the doping of rare-earth chromites was carried out by introducing the bivalent metal ion component stoichiometrically as a solid into the initial aqueous solution as a substitution for some of the rare-earth oxide.
This method requires the preparation of numerous gradations of rare-earth chromite powder having differing electrical resistivity characteristics, which may then be formed into shaped electrical elements by ceramic forming techniques for specifically desired applications.
Each group of similar elements must be produced separately from separate batches of modified rare-earth chromite powders prepared specifically for the electrical resistivity characteristics desired.
As used herein, the word "elements" refers to electrically conductive ceramically formed bodies of various shapes which may be used as semiconductors, resistors, electrodes or the like.
Also, in certain specific electrical applications, such as the production of thermoelectric elements, it has been found that elements having uniform resistivity are subject to rapid deterioration at certain points when exposed to electric currents. More specifically, such elements are generally provided with electrical contact points for connecting the element to an electric current. The electrical contact points of such an element often overheat if the element has an overall high resistance. Particularly in environments requiring a thermoelectric element capable of attaining very high heat values, problems arise in preventing the destruction or rapid deterioration of the electrical contact points of the element from the excessive heat.
This problem has been reduced in significance by the use of elements having differing resistivity at different portions of the element, such as high resistance at a heating element portion and low resistance at electrical contact points.
Elements having differing resistivity at different portions of the element may be formed using a layering technique with graded powders as above, described in U.S. Pat. No. 3,531,421 to Foex et al. However, such a method becomes less economical as the required number of different rare-earth chromite powders and the number of specifically designed elements increases.
It is therefore a primary object of the invention to provide an improved method for reducing the electrical resistivity of a rare-earth chromite material.
Another object of the invention is to provide an improved method for forming a rare-earth chromite element having varying electrical resistivity at different portions of the element.
Additional advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the present invention may be realized and attained by means of the combinations particularly set out in the appended claims.